This invention relates to an extrusion type coating apparatus and particularly to a coating apparatus capable to carrying out a coating operation without trouble even under the high-speed coating conditions when manufacturing magnetic recording media.
As for the coating systems, there have been well known various systems including a roll-coat system, a gravure-coat system, an extrusion-coat system, a slide-bead-coat system, and a curtain-coat system.
Magnetic recording media may be obtained by coating a magnetic coating solution on a support. The coating systems applicable therefor include, generally, the roll-coat, gravure-coat and extrusion-coat systems. Among these systems, the extrusion-coat system is superior to others, because a uniform thickness of coated-layers can be obtained.
In recent years, on the other hand, magnetic recording media have been rapidly improved, and oxidized magnetic powder with a high BET value and barium ferrite materials have been used so that coating solutions have been highly thickened. Further, with the purpose of improving productivity, there have been increasing demands for high-speed coating operations.
As for the extrusion-coat systems mainly for preparing magnetic recording media, there are known conventional techniques such as those described in Japanese Patent Publication Open to Public Inspection (hereinafter referred to as Japanese Patent O.P.I. Publication) Nos. 57-84771/1982, 58-104666/1983 and 60-238179/1985.
In recent years, multi-layered magnetic recording media have been tried, because a higher density and thinner layers or a more data storage was desired. To meet these requirements, there have been proposed double-layer coating apparatuses such as that described in Japanese Patent O.P.I. Publication No. 63-88080/1988.
In the above-mentioned extrusion coat systems, a uniform thickness of coated layers can be obtained. However, satisfactory coating conditions may be obtained only within a limited range. Accordingly, there may be often instances where a desirable coating operation may hardly be made under the above-described high-speed coating conditions.
The serious difficulties which arise in coating operations and particularly in thin-layer coating operations (for making a layer not thicker than 30 .mu.m) include, for example, a coated area is blanked out by foreign material getting caught or adhering to a support; dust or other coagulated substances in the coating solution depositing on the back edge surface; some portions of the layer being coated being thicker than others, especially in the longitudinal direction; and scraping of the support by the front edge portion, especially by the square portion of the downstream end thereof, thereby producing shavings which adhere to the apparatus. Particularly when coating a very viscous solution, cross-streaking is likely due to undersired movement of the support, thereby producing noise or fluctuations in output.
To solve the above-mentioned difficulties, various countermeasures have been devised. A typical example thereof is the technique described in Japanese Patent O.P.I. Publication No. 60-238179/1985. (hereinafter referred to a Prior Art 1)
In Prior Art 1, bubbles or pinholes caused by coating are moving, air is prevented from being sucked into gap between the surface of a support and a front edge surface by squeezing the downstream end of the front edge surface.
In this Prior Art 1, however, the travelling angle of a support is sharply changed at the downstream end of the front edge surface, so that the contact pressure of the support on the downstream end thereof may be concentrated at the downstream end however, the support surface is scraped by the downstream end, so that there may be high frequency problems caused by shavings of the support on the coated surface.
On the other hand, the double-layer coating apparatus cited in Japanese Patent O.P.I. Publication No. 63-88080/1988 (hereinafter referred to as Prior Art 2) is also based on the same idea as in the above-described Prior Art 1, so that the adhesion of the support base shavings may not be avoided.
In other words, in the above-cited Publication, it is regarded as preferable that, in FIG. 4, .theta..sub.1, .theta..sub.2 and .theta..sub.3 each have the following relationship: EQU 0.5.degree.&lt;.theta..sub.1 .ltoreq.35.degree. EQU 0.degree.&lt;.theta..sub.2 .ltoreq.25.degree. EQU 0.degree.&lt;.theta..sub.3 .ltoreq.20.degree.
However, if the above relationships are observed, the device is likely to produce base shavings at downstream end X of a front edge surface and to scrape the first layer coating solution away from the downstream end of a center edge surface.
Even though the above-mentioned problems may be partly solved by reducing the contact pressure of the head surface of a coating apparatus on a support, the contact pressure should be increased to some extent when a high-speed coating is to be stably made. In this case, the above-described problems still remain unsolved.
It is therefore a principal object of the invention to provide a coating apparatus capable of firmly preventing a difficulties in base shaving adhesion and a bubble or pin-hole formation in a support surface caused by sucking the air at the interface between layers.